Prevent duplicate vendor profiles from being created

We have a frequently occurring issue: vendors who already have a profile in our system will create a duplicate profile with a new email address, either because they forgot they already had an account, or because they didn't understand the system, or whatever other reason. Preventing this from happening seems very difficult; I don't know what criteria could be used to block existing users from creating a new profile, because they're using a different email when they do it; and trying to educate vendors on the subject isn't really viable, because we're not in contact with most of them until after the issue occurs. Anyway, we wanted to ask the Echo community and XTRF experts if there's anyone else who has experienced the same issue, and any ideas they have.

Best,

David